Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Mon Dec 2 2024 Valid 12Z Thu Dec 05 2024 - 12Z Mon Dec 09 2024 ...Another blast of Arctic cold trough the central and eastern U.S. later this week... ...Heavy Great Lake effect snows to continue through Thursday... ...Overview... Upper level troughing settled over the central and eastern U.S. will dominate the weather pattern through this week into next weekend, and weather systems will be driven by shortwave troughs rotating around the broad upper level trough. A couple cold fronts will push south across the eastern two-thirds of the nation this week and will bring another blast of cold Arctic air. This pattern will also favor lake effect snow in the Great Lakes, but the heaviest should come to an end mid to later this week. Elsewhere, high pressure will persist in the interior West before a frontal system works through next weekend. Moderate rains will focus over the Pacific Northwest later this week, with snow chances spreading inland across especially favored terrain from Cascades through the Northwest and northern Rockies next weekend with system/colder flow progression. Meanwhile, moist return flow from the Gulf will support daily enhanced rain chances set to spread from the western to central Gulf Coast states and parts of the Southeast. ...Guidance/Predictability Assessment... Guidance still shows fairly good agreement through much of the period on the overall large scale flow, albeit in a pattern with closer to average overall predictability given increasing embedded system forecast spread through next weekend. Confidence is above normal with respect to a main cold front through the East late this week, ushering in another round of Arctic air for parts of the Northern Plains, Midwest, and into the East. The next shortwave into the north-central U.S. on Friday should be weaker, but does show some uncertainty in terms of timing. Out West, the guidance shows the upper ridge shifting east next weekend to usher in a more cool and unsettled pattern with some lingering uncertainty in the timing of a main/wavy cold front to work over the Northwest/West. Accordingly, the WPC medium range product suite was mainly derived from a composite blend of best clustered guidance of the 18 UTC GFS and 12 UTC ECMWF/Canadian along with compatible 01 UTC National Blend of Models and WPC continuity for Thursday/Friday. This solution also has strong multi-center ensemble support and favorable trends from newer 00 UTC guidance including a now more in line 00 UTC UKMET. Manual adjustments to guidance was applied to more strongly emphasize Great Lakes snow potential lingering this period given favorable support. Later opted to switch deterministic model preferences in the blend to their respective GEFS/ECMWF/Canadian ensemble means starting next weekend amid steadily growing embedded system forecast and continuity variances. Latest 00 UTC deterministic model guidance also remains less than stellar with run to run continuity with embedded systems at these longer time frames. ...Weather/Hazards Highlights... Lake effect snow is forecast to continue this week as a clipper low pressure system moves across the region Wednesday and Thursday followed by strong west-northwesterly winds. Significant snow accumulations will be possible in favored downwind locations, and travel conditions will likely be hazardous, especially earlier in the period. Snow will also be possible elsewhere in the Northeast as the clipper system moves through, especially in the higher terrain of the Appalachians. Conditions should improve briefly late this week, but another low pressure system will bring another chance for wintry precipitation to the region next weekend. The rest of the nation will be mainly dry this week with a couple exceptions. Moist return flow from the Gulf will result in daily rain chances along the Gulf Coast and into portions of the South. While there will likely be some instability to support locally heavier downpours, no threat areas are depicted in the WPC Excessive Rainfall Outlook. Any risk of heavy rainfall will decrease through the end of the week as instability decreases and precipitation becomes more spread out across the Gulf Coast region as fronts move through. Precipitation chances will also return to the Northwest by next weekend as a Pacific frontal system approaches and works steadily inland. This will act to produce a more cooler and unsettled airmass across much of the West, with moderate rains over the Pacific Northwest and some terrain enhancing snows inland across the Northwest and northern Rockies. Temperatures will drop below normal for parts of the north-central to eastern U.S. this week as a cold front ushers in another blast of Arctic air. Daytime highs from the Upper Midwest into the East could be 10-20 degrees below normal. Elsewhere, temperatures will be mild for this time of year, generally near to slightly above average with this milder air gradually expanding east with time. Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw